- Combinaciones de negocios

Primary Well Barrier Secondary Well Barrier

Statically underbalanced fluid column

In-situ formation

Casing* Casing cement

Wellhead* Casing*

High pressure riser* Wellhead*

Drilling BOP* High pressure riser* Rotating control device Drilling BOP* UBO/MPD non-return valve

Drill string or completion string UBO/MPD choke system

In addition, the concept “common well barrier element” is introduced (marked with * in Table 4- 2), meaning that the element is shared between the primary – and secondary WB. [7, p. 10] Operating with common well barrier elements require a risk analysis and risk reducing measures to be applied. One must also be aware that a failure of a common well barrier element means failure of both primary and secondary well barrier element. During the MPD operation at Gullfaks, a leakage in the 13 3/8’’ casing occurred, leading to failure of both primary and secondary well barrier. See chapter 6.

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4.2 W

C

Establishing and maintaining control of the well is of major importance during all drilling – and well activities. Well control is a collective term, expressing all measures (including procedures, practices, equipment, etc.) that can be applied for preventing a well control scenario, i.e. uncontrolled release of well bore fluids. [7, p. 15]

In order to maintain well control, monitoring of pressure and volumes is required. As described in Chapter 2, the result of operating with pressure below the formation pore pressure or above formation fracture pressure is an uncontrolled flow situation, i.e. control of the well is lost. 4.2.1 WELL CONTROL ACTION PROCEDURES [7]

Procedures shall be available for all drilling – and well activities (see chapter 5.3). The purpose of well control action procedures is to provide a plan for activating well barriers, and how to react in case of a well control situation. Ahead of any operation, the operator and contractor shall ensure that all involved personell are familiar with the well control action procedures, and a well control bridging document shall be prepared between the disciplines.

According to NORSOK D-010 (2013), the well control briding document shall define roles and responsibilities related to well control during the operation, shut-in procedures, various methos for how to re-establish well barriers (including activation of alternative WBEs, kill procedures and normalization), and specific well control conficuration for the well activity. [7, p. 29]

Well control action procedures shall be developed for potential incident scenarios related to the specific operation. For conventional drilling operations, potential incident scenarios include, among others, shallow gas influx and influx containing H2S. [7, p. 42]

In addition, there must be developed action procedures depending on the position of the pipe or tools if an influx should occur (i.e. shearable pipe, non-shearable pipe or no pipe through the BOP). It is of major importance that the operator (i.e. driller) of the BOP system is aware of the position of the various drill string components in relation with the various BOP rams. For example, activation of the pipe ram (designed for sealing the dimension of the drill pipe body), will most likely fail if a tool joint is positioned in the closure area.

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Introducing MPD, with related equipment and operation, the main operational risk shall be identified, and included in the well control action procedures. The additional elements are primarly due to the use of common well barrier element, leaking or plugging of MPD equipment, and failure of keeping constant bottom hole pressure. Loss of rig power is critical for all drilling acitivites, but particulary when operating with narrow drilling window.

4.2.2 WELL CONTROL ACTION DRILLS

Preventive measures include action drills performed by the drilling crew. Action drills are of major importance for familiarizing the involved personnel with techniques and procedures that may be relevant in case of a well control scenario.

For conventional drilling operations, the action drills that should be performed includes kick drill while drilling or tripping (depending on the operation), choke drill and H2S drill, among others. The frequency depends on the drill to be performed, where for example kick drill while tripping (common operation) shall be performed once per week per crew, whereas the H2S drill shall be performed prior to drilling into a zone/reservoir that may contain H2S (i.e. depending on the formation and fluids). [7, p. 43]

These action drills are performed by simulating the various well control incidents, for example taking in a kick while drilling. Local procedures are available for execution, and involved personnel have different tasks during the drills (e.g. driller, roughneck).

The additional well control action drills for MPD operations are in large degree a result of introducing new equipment. Examples of action drills that should be performed include leaking NRV and leak in RCD. In addition, practice related to transferring between conventional and MPD is of major importance, and should be performed once prior to start the MPD operations with the crew on location. [7, p. 141]

In addition to well control action procedures and well control action drills, a well control matrix shall be prepared ahead of MPD operations, defining criteria for stopping the operation and how to respond to the situation.[7, p.141] It is important to develop review and update the matrix ahead of each operation, as new elements (for example downhole pressure conditions) or

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equipment may be present. All personnel involved in the operation must be well familiar with the MPD matrix, for ensuring a safe execution.

4.2.3 WELL CONTROL SCENARIO

Typical indicators for taking in a kick, include, but are not limited to [10] - Well flowing with the pumps off

- Changes in drilling rate

- Increase in the mud flow from the well - Pit gain

- Decrease in pump pressure and increase in pump speed - Increase in rotary torque

If changes in drilling parameters should occur or a suddenly increase in ROP (known as drilling break), it is normal procedure for the driller to perform a flow check in order to verify if the changes is caused by “false alarm” or small fluctuations, or if it is actually a well control scenario. - Flow check: “A test performed to ensure stable well conditions or the integrity of a plug,

valve or flow-control device. In most cases, the flow check involves observing stable fluid levels or conditions for a prescribed period.” [25]

Table 4-3 illustrates the procedure for performing a flow check, either conventionally or during MPD operation.